I've done a bit of searching about this but can't find a recent answer. I'm using Calibre to create a mobi out of an epub as I get better results than via kindlegen but I can't seem to get the thumbnail to appear in the Kindle PC app, though the thumbnail shows up fine on the device and the cover itself is viewable in large on both the app and the device. The kindlegen version creates a viewable thumbnail but I don't use that version as I get weird formatting problems with that.
I hear that it's probably a bug of some sort, but has there been any resolution to it yet?

@mo79: Try doing a Calibre convert *with* the 'Enable sharing of book content' setting checked. If it doesn't help, try again (with your usual settings first, and then with the 'Enable sharing' if it still doesn't help) after applying this patch to Calibre.

Cheers - that worked! Finally looked at the Calibre FAQ, and if this is a K4PC bug I'm happy to leave it really.
I notice that kindlegen by default also produces the .mobi as a personal document which is why the Calibre conversion works as it apes that.

Technically, the key here is not so much the personal document stuff, but the fact that there's an ASIN specified in the exth headers (there isn't one when kindlegen builds a mobi, but Calibre adds a 'fake' one to support Syncing, among other things). If there is, Amazon assumes the file comes from them, and will try to use it to get the corresponding thumbnail from their servers. If it's bogus, it fails horribly .

The fact that K4PC/Mac/Android think files without an ASIN are PDOCs is another thing entirely. If you just happened to mark a calibre file with an ASIN as pdoc (through the cdetype exth header), it would still fail to show a thumbnail, although it technically is marked as a PDOC (not sure how K4PC would sort such files, but the e-Ink devices correctly mark them as pdocs).

@mo79: KindleGen/KindlePreviewer doesn't put an ASIN in the headers, and properly reference the thumbnail, so, yeah, it works . The Calibre FAQ answers the first point (how not to end up with an ASIN in Calibre generated files), while my patch tries to address the second point (which seems to be moot anyway on some devices/apps if there's a bogus ASIN in the headers).